auto-complete back-end for RTags
--------------------------------

sample configuration for `.emacs`

    (use-package ac-rtags
      :config
      (setq rtags-completions-enabled t)
      (add-hook 'c-mode-common-hook '(lambda ()
                                       (setq-local ac-sources (append '(ac-source-rtags) ac-sources))
                                       (auto-complete-mode))))